Nonpolar filaments that are generally found only in multicellular organisms. These intermediate sized (10 nm in diameter) filaments are thinker than their microfilament analogs, but thinner than microtubules, thus the name "intermediate". Intermediate filaments are structural in function, and pass through cell junctions and intermembrane proteins to pass through several cells and integrate them into tissue (sort of like a wire mesh that provides larger structure and support).

The strength of intermediate filaments is illustrated by claws and hair, which are made up of dead cells that are composed mainly of intermediate filaments. This stability extends to the chemical realm as well. Intermediate filaments are not dynamic in the way the microtubules and microfilaments are. Instead of constantly stacking and unstacking, it is formed by the relatively stable polymerization into a rope-like structure.

Although IFs are not as dynamic as other filaments,and there are no motor proteins that are known to work with them, they are even more unusual in that they are composed of 6 (plus or minus depending on how you clasify) different types of subunits. The first two types (acidic and basic keratins) combine with each other in various ways (though always 1 acidic to 1 basic) to form the various types necessary for epithelial cells (from that in hair, to that in the lumen the intestine).
The third type do not have to form with any other type (they can form "homopolymeric" IFs), but they can. This type is found in muscle and glial cells, as well as neurons. The type three Desmin is particularly important in stabilizing the z disk of sarcomeres.
The fourth type is found only in the nervous system and are responsible for axon elongation.
The fifth type is found only in lens fiber cells.
The final type is found only in the nucleus (of all eukaryotes) and make up what is commonly known as the "nuclear lamin" which is a network that supports the nuclear membrane (the role of intermediate filament associated protein (IFAP) receptor for lamin B in binding lamin B to the membranes particularly interesting).

Mutations affecting the expression of intermediate filaments can lead to horrible genetic skin blistering diseases like epidermolysis bullosa simplex.

A type of structural protein found within the cytoplasm of eukaryotic cells which forms one type of cell cytoskeleton. In mammalian cells, the intermediate filaments are intermediate in width between actin (a muscle protein) and microtubules. There are six types of intermediate filaments: keratins, desmins, vimentins, glial filaments, neurofilaments, and nuclear lamins.

From the BioTech Dictionary at For further information see the BioTech homenode.

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